Basic interphase transformers of the nature illustrated in FIG. 1 have been known for some time for paralleling outputs of multi-phase power electronic circuits employed in power conditioners. Such basic interphase transformers, however, exhibit significant output current flux fringing. As indicated in FIG. 2, modified interphase transformers employing two C-shaped magnetic core halves 21 with windings 20a and 20b and air gaps between the laminated steel core halves are also well known. Such modified interphase transformers with inputs 22 and 23 along with output 24 offer the advantage of minimizing the main output current fringing flux.
These conventional modified interphase transformers when properly designed are well suited for paralleling six phase (dual three phase) sources feeding power conditioners such as cycloconverters. However, important design parameters for said systems must always take into consideration system weight, size, volume and, of course, cost. Although, as aforementioned, the above noted conventional interphase transformers serve well for the paralleling of six phase outputs, the required number of such interphase transformers increases significantly when inputs of a nine, twelve or even higher number of phases are involved. Such increases in the number of transformers are required for balanced filtering, paralleling of output current for controlling the level of circulating current and commutation of thyristors or the like.
Where, for example, a system involves nine input phases, three interphase transformers are necessary. Moreover, the number of interphase transformers increases to six for a system involving twelve input phases. The mathematical rule is that the combination of the number of three phase groups taken two at a time equals the required number of interphase transformers per output phase. Similar increases in the number of interphase transformers are necessary for still further increases in the number of input phases. Clearly, such significant increases add substantially to system weight, size, volume and cost.
An additional factor adding to the system complexity and costs where nine or more phases are to be paralleled are not only the number of additional interphase transformers required for balanced filtering and commutation, etc., but also the number of connections that are necessary among the elements. For example, for a nine phase system as schematically illustrated in FIG. 5, an arrangement of three interphase transformers with inputs from each of the three three phase groups 41, 42 and 43 results in a total of nine connections per output phase, including the common connections. A system involving three output phases with a nine phase input source thus requires three arrangements of the nature shown in FIG. 5. Such an arrangement has the disadvantages of a reduction in balanced filtering, good paralleling, as well as unbalanced commutation coupled with more complexity and an increased number of connections.
My invention involves the discovery that the advantages of the prior art arrangements; namely, minimum flux fringing, balanced filtering and paralleling, as well as commutation of a six phase system, may be extended to a system with more than six phases through the use of a single multi-interphase transformer per output phase. Although my multi-interphase transformer optionally can be designed for any number of phases of the source (preferably a multiple of three), it is particularly advantageously used in any power conditioner having an input power source with more than six phases and requiring an interphase transformer.
As described in more detail below, the multi-interphase transformer of the present invention includes three or more C-shaped cores each including a winding with one end of each winding commonly connected to an output phase terminal and with the other ends of each winding connected to an output lead of a power conditioner fed by polyphase sources. Air gaps among the core assemblies are formed between angled planar surfaces at the ends of the C-shaped cores. Selection of appropriate clearances and other magnetic circuit parameters for obtaining appropriate low frequency and end-to-end inductances allows application to a wide range of power conditioning systems without adversely affecting filtering and commutation balance. Moreover, the requirement of only one multi-interphase transformer per output phase allows a system design with minimum system weight, size and volume. Additionally, since fringing flux due to the main output current is minimized, other components, as well as the system chassis, may be positioned closer to the multi-interphase transformer than would be required for other prior art designs.
Thus, the principal objective of my invention is that of extending the advantages of the above noted conventional modified interphase transformers of FIG. 2 (ideal for a six phase source) to power sources with more than six phases, while additionally eliminating the disadvantages of the conventional systems of FIG. 5. In this regard, the exemplary embodiment disclosed herein may serve as a replacement device in a polyphase system for a number of conventional modified interphase transformers, as well as replacing and further reducing the number of required connections conventionally found in interphase transformer systems employed with power sources with more than six phases.